The Fed has emerged as one of Wall Street’s biggest customers during the financial crisis, buying massive amounts of securities to help stabilise the markets. In some cases, such as the market for mortgage-backed securities, the Fed buys more bonds than any other party.

However, the Fed is not a typical market player. In the interests of transparency, it often announces its intention to buy particular securities in advance. A former Fed official said this strategy enables banks to sell these securities to the Fed at an inflated price.

The resulting profits represent a relatively hidden form of support for banks, and Wall Street has geared up to take advantage. Barclays, for example, e-mails clients with news on the Fed’s balance sheet, detailing the share of the market in particular securities held by the Fed.